A large number of Rankine cycles for the production of mechanical and then electrical power from a single thermal source have been highly developed over the past century. Over the past four decades, a large number of variations have been described and evaluated for the purpose of improving the economic utilization of low-grade heat, as available from most geothermal sources, typically in the range of 360 K to 450 K, or mid-grade heat, as available from concentrated solar power (CSP), typically in the range of 480 K to 730 K.
Many geothermal projects have utilized an organic working fluid such as isobutane, as it has a fairly high vapor pressure at the typical condensing temperature (˜300 K) and has a relatively low latent heat of vaporization at the typical boiler temperature, ˜400 K. Some have utilized multi-component fluids, including propane/ethane mixtures, and some have used synthetic refrigerants such as R-22B1, CHBrF2, or ammonia, NH3.
For mid-grade heat sources, cascaded cycles have recently been utilized in which a higher boiling fluid, such as benzene, water, or toluene, is heated to the maximum available temperature; and its condenser, typically near 430 K, drives the boiler for a loop utilizing a lower-boiling fluid such as isobutane. Pressure ratios are typically about 10 in each loop, and recuperation is also usually utilized, as the expander temperature ratio (TR=TIT/TOT, turbine inlet temperature divided by turbine outlet temperature) is only about 1.15 in such fluids—because γ, the ratio of CP to CV, is under 1.1 at the typical expander conditions. Others have utilized mixtures of ammonia and water that are at some points in the cycle mixed and at other points separated in ways that in principle permit the major heat transfers to take place with reduced temperature differences and hence improved efficiency, albeit with considerable increase in complexity, mass, and cost. These have all been designated as Organic Rankine Cycles (ORCs), as distinguished from conventional water steam cycles.
The recent trend toward higher peak temperatures has pushed the fluid choice toward aromatics, such as benzene and toluene, because of their very low susceptibility to dehydrogenation. However, their low vapor pressures at ambient temperatures require the use of costly, cascaded cycles to avoid sub-atmospheric systems (which lead to ingress of air and moisture through unavoidable minute leaks).
Widespread misconceptions related to chemical stability include the notion that higher boiling points generally correlate with high thermal stability and that the upper temperature limit is mostly determined by the fluid choice. We disclose herein methods for increasing the practical temperature limit for light alkanes by 200-350 K primarily from the combination of (a) accommodation of hydrogen evolution, (b) minimization of high-temperature residence time, (c) deactivation of catalytically active surfaces, (d) incorporation of on-line membrane separations processes, (e) increasing the condensing pressure, and (f) choosing a more optimum fluid mixture.
The latent heat of vaporization of the working fluid and the differences in specific heats between the liquid and vapor phases make full optimization (closely approaching second-law limits) impossible for a single heat source with known working fluids. When two heat sources are available, this problem may be effectively solved. We disclose herein a method of achieving much higher overall efficiency by using a combination of a mid-grade source (such as CSP) and a low-grade source, such as geothermal, industrial waste, low-grade solar concentrators, low-cost flat-plate solar collectors, or oceanic thermal gradients.
A Recent CSP ORC Example. A recent (2005) economic analysis of a cascaded ORC for a field of solar troughs (Prabhu, US-NREL/SR-550-39433, 2006), estimated the installed cost of just the 5 MWPE power plant would be over $3/WPE, where WPE is the peak electrical power output. This cost is an order of magnitude beyond what is needed to be economically viable in most cases—especially for solar, where the average power is usually under 28% of the peak power. In this study, about two-thirds of the cost was the installation cost. The highest peak net ORC efficiency predicted in this study for a source temperature of 663 K was 30.5%, which, though 50% higher than seen in some other recent ORC examples, is still about 55% of second-law theoretical limits with a 300 K sink. However, average efficiencies throughout the year are typically 8-18%, as both ORC and collector efficiency decrease from mid-day performance.
Reducing the cost by an order of magnitude will require making the power plant compact enough to be easily transported from the factory to the field site by truck in a reasonable number of easily separable modules after check-out at the factory. In the above study, the 5 MW power plant used an area of approximately 5,000 m2. Factory production of such a large power plant is completely out of the question. The size needs to be reduced by one to two orders of magnitude.
Overview of the HT Dual-source ORC (HT-DORC). The current invention has two main components: (1) a method of efficiently utilizing heat from two separate sources (one of lower grade and one of higher grade) to allow significantly higher total efficiency and reduced system cost; and (2) a method of substantially extending the upper temperature limit of exemplary working fluids (those with vapor pressure greater than 0.1 MPa at ˜270 K and having thermal conductivity greater than 0.035 W/m-K at 500 K), primarily by accommodating the evolution of hydrogen and minimizing HT residence time.
With respect to the dual-source feature, the novel approach utilizes recuperation in the Rankine cycle to the extent practical (as partially limited by the thermodynamic properties of the working fluid), but with most of the heat of vaporization and some of the liquid preheating being provided by a low-grade heat source while the final superheating is provided by a mid-grade or high-grade heat source.
There are three fundamental advantages to the highly-recuperated DORC. In order of generally decreasing significance, these are:    (1) The boiling temperature can be greatly reduced with no adverse affect on efficiency of utilization of the higher-grade (more expensive) heat source (provided the boiling enthalpy is available from a low-cost heat source). This allows the use of fluids having higher thermal conductivity and higher vapor pressure at the condenser temperature, which allows for reductions in the size and cost of the expander, recuperators, and condenser.    (2) Only one expander turbine is required to approach theoretical efficiency limits, and its size is reduced (because of the higher condenser pressure and lower molecular mass of the working fluid).    (3) The working fluid mixture and the pressures may be selected such that temperature differences in all the heat exchangers may be more fully minimized at all points in the cycle.
The optimum working fluid would (a) have at least 0.1 MPa vapor pressure at the minimum condenser nighttime temperature (generally between 250 K and 285 K), (b) have excellent chemical stability in the super-heater, (c) be sub-critical near the temperature of the lower-grade heat source, (d) be environmentally safe, (e) have high thermal conductivity in the vapor phase, (f) have high autoignition temperature, and (g) have high γ. Most prior discussions of optimum fluid selection have focused largely on only one of the above criteria, or on another—the slope of the saturated vapor line on the T-S diagram, which is now irrelevant in the DORC.
Reducing the Size and cost of ORCs. The most important innovation for reducing cost is to get the most out of a single thermodynamic loop and a single turbine expander (by far the most expensive single component in prior ORCs) while still fully minimizing temperature differences in all the exchangers. Cascaded loops have been chosen in the past to avoid dehydrogenation while still keeping the condenser pressures above 0.1 MPa. We show that it is much better to instead optimally address the chemical stability problem.
The second most important change is to increase the condenser pressure. This is essential for reducing the size of the condenser and recuperators, where relative pressure losses scale inversely with the square of the pressure. Increasing the condenser pressure is also beneficial in improving chemical stability of the working fluid and in simplifying the separation of light gases (H2, CH4, C2H6, etc.) from the vapor stream, which we show to be essential for dramatically increasing the temperature limit. The use of higher condenser pressure, lower pressure ratio, and improved HT recuperation allows an order-of-magnitude reduction in the cost of the single expander turbine needed in the DORC.
The third most important requirement for reducing the power plant size and cost is to use ultra-compact recuperators. Gas-to-gas recuperator designs that are more than an order of magnitude more compact than in the referenced example have been well known for two decades.
For solar CSP, increasing the efficiency of utilization of the higher-grade heat source is actually the most significant factor in reducing total system cost, as the cost of the solar field is often three times the cost of the ORC. The DORC allows this efficiency to be nearly doubled with moderate temperature increase. Solar concentrators have achieved temperatures above 1500 K so a significant increase in collector temperature (compared to 660 K) without much increase in radiation losses should be straightforward. However, the fluid used to transfer heat from the solar field must have a much higher boiling point, and the chemical stability of the fluids must be dramatically improved. Solutions to these issues are presented.
Finally, it is necessary to improve off-design performance so that the cost of thermal storage can be greatly reduced. Measures for improving off-design performance are disclosed.
Removal of Reaction Products. In the conventional ORC, the loss of expansion ratio that results from a non-condensable partial pressure in the condenser has a very detrimental effect on expander shaft power and efficiency, as the increased turbine outlet enthalpy is not usable. In the DORC, where recuperation above the boiling point is very effectively utilized (as will be seen), an increased turbine outlet temperature means that less final superheating is required. Hence, the efficiency of utilization of the heat sources is hardly effected by high H2 partial pressure in the condenser. The mass flow rate of the working fluid must be increased for a given output power, and the expander must continue to work efficiently at a lower expansion ratio; but these are minor technical issues. While it is still preferable to maintain fairly low H2 partial pressure in the condenser, high enough H2 partial pressures are acceptable that the task of separating light-gas reaction products in the DORC becomes practical. Several methods for achieving the needed removal of reaction-products (both light and heavy) from the working fluid are disclosed.
Applications for DORCs. There are a number of very important (and neglected) cases where substantial amounts of low-grade and mid-grade waste heat may be available simultaneously. Fischer-Tropsch Synthesis (FTS) reactors reject hundreds of megawatts at 500 K to 650 K, and lesser amounts are rejected in condensers at lower temperatures. A wind-electrolysis-fueled FTS process is the subject of another pending patent application. There, amounts of heat greater than the FTS reaction are also rejected from the water electrolysis at 400 K to 440 K, and perhaps eventually at up to 500 K.
Excellent solar resources are often present near many good geothermal resources. In such cases, much more economical resource utilization can be achieved by using a DORC with the geothermal resource driving the boiler (perhaps near 400 K) in combination with a concentrated solar super-heater at 650 to 820 K. Prior geothermal ORCs have usually achieved 10-14% thermal efficiency, and prior concentrated solar ORCs have generally achieved 20-32% efficiency. The isobutane DORC can exceed 27% electrical conversion efficiency of the total thermal input (low-grade plus mid grade), and the electrical output may exceed 55% of the more expensive, mid-grade (CSP) contribution.
Vertical oceanic thermal gradients in some bays can reach 25° C. within 150 m of depth (though usually the gradients are much less), and there have been some attempts to utilize these gradients to generate electrical power using various ORCs in what is called Oceanic Thermal Energy Conversion (OTEC). Such attempts have achieved only 1% to 3% thermal efficiency, and thus have not been economical. However, in most cases where such oceanic thermal gradients are found, the local solar resource is also excellent. Hence, a much more cost effective engine can be made by using a DORC with a condensing temperature of 285 K (a little above the deep-water bay temperature), a boiler temperature at 300 K (a little below the surface water temperature), and a concentrated solar super-heater at ˜750 K. The low-grade boiler heat allows one to obtain about 50% efficiency in conversion of the mid-grade solar energy, or perhaps 10% conversion of total thermal input.
Flat-plate solar collectors and low-grade solar concentrators provide low-quality solar heat at much lower cost per GJ than the mid-grade energy from high-temperature CSP. The combination of flat-plate collectors or low-grade solar concentrators providing the boiling enthalpy at 350-480 K with CSP providing the final superheating at 650-800 K promises higher cost effectiveness for renewable electricity than any other solar option currently on the horizon.
The DORC would also allow much higher efficiency in combined-cycle fossil-fuel power plants located near geothermal sources. The steam condensing temperature could be higher, perhaps 400-450 K, to reduce the cost of this steam cycle heated by the exhaust from the fossil-fueled turbine. The steam condenser could provide the mid-grade heat to the DORC, with geothermal providing its low-grade heat.